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ScienceDirect Physics Procedia 87 (2016) 54 – 60

44th Annual Symposium of the Ultrasonic Industry Association, UIA 44th Symposium, 20-22 April 2015, Washington, DC, USA and of the 45th Annual Symposium of the Ultrasonic Industry Association, UIA 45th Symposium, 4-6 April 2016, Seattle, WA, USA

Airborne Power Ultrasonic Technologies for Intensification of Food and Environmental Processes Enrique Rieraa*, Víctor M. Acostaa, José Bonc, Manuel Aleixandrea, Alfonso Blancoa, Roque R. Andrésa, Andrea Cardonib, Ignacio Martinezb, Luís E. Herranzd, Rosario Delgadod, Juan A. Gallego-Juáreza,b * a

Dpto. Sensores y Sistemas Ultrasónicos, ITEFI, CSIC, Serrano 144, E28006-Madrid, Spain b PUSONICS S.L., Pico Mulhacen, 34,E28500-Arganda del Rey, Madrid, Spain c Dpto. Tecnología de Alimentos, Universitat Politécnica de Valencia, Camino de Vera s/n, E46022-Valencia, Spain d Unidad de Seguridad Nuclear, División de Fisión Nuclear, CIEMAT, Avda. Complutense, 22, E28040-Madrid, Spain

Abstract Airborne power ultrasound is a green technology with a great potential for food and environmental applications, among others. This technology aims at producing permanent changes in objects and substances by means of the propagation of high-intensity waves through air and multiphase media. Specifically, the nonlinear effects produced in such media are responsible for the beneficial repercussions of ultrasound in airborne applications. Processing enhancement is achieved through minimizing the impedance mismatch between the ultrasonic radiator source and the medium by the generation of large vibration displacements and the concentration of energy radiation thus overcoming the high acoustic absorption of fluids, and in particular of gases such as air. Within this work the enhancing effects of airborne power ultrasound in various solid/liquid/gas applications including drying of solid and semi-solid substances, and the agglomeration of tiny particles in air cleaning processes are presented. Moreover, the design of new ultrasonic devices capable of generating these effects are described along with practical methods aimed at maintaining a stable performance of the tuned systems at operational powers. Hence, design strategies based on finite element modelling (FEM) and experimental methods consolidated through the years for material and tuned assembly characterizations are highlighted. ©©2016 Authors. Published Elsevier B.V.access article under the CC BY-NC-ND license 2016 The Published by Elsevier B.V.by This is an open Peer-review under responsibility of the Ultrasonic Industry Association. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Ultrasonic Industry Association.

* Corresponding author. Tel.: +34 915628806; fax: +34 914117654. E-mail address: [email protected]

1875-3892 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Ultrasonic Industry Association. doi:10.1016/j.phpro.2016.12.010

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Enrique Riera et al. / Physics Procedia 87 (2016) 54 – 60 Keywords: Ultrasonic processing; power ultrasound; mass transfer; drying; acoustic agglomeration; nuclear safety; source term mitigation.

1. Introduction Airborne power ultrasound consisting in the generation and transmission of high-intensity pressure waves in air at frequencies in the range 20 to 50 kHz is considered an attractive emerging technology with a great potential in food and environmental applications. The recent development of specific ultrasonic technologies based on power piezoelectric transducers driving extensive plate radiators, described by Gallego-Juárez et al. (2010), Riera et al. (2011) and Gallego-Juárez et al. (2015), lead to a renewed academic and industrial interests in the applicability and scalability of airborne power ultrasound technology in multiple operations. The reasons behind the interest in these novel transducers are their electro-acoustic efficiency (up to 80%), high energy concentration (high directivity and/or focalization), power capacity (up to 0.5-1 kW) and ability to generate large vibration displacements in air. The technology consists of an ultrasonic vibrator, constituted by a Langevin piezoelectric transducer and a mechanical amplifier that drives an extensive plate radiator in a flexural mode (Gallego-Juárez et al., 1978) generating highintensity ultrasonic waves. These waves propagate through air, gases or aerosols producing a series of linear and nonlinear effects such as co-vibration, entrainment, radiation pressure, high amplitude compressions and rarefactions, diffusion enhancements, turbulence and acoustic streaming as described by Riera et al., (2015). Since 2008 the CSIC and Pusonics SL have been working together to introduce airborne ultrasonic technology into laboratories and industries by designing customized ultrasonic devices with enhanced radiation characteristics. As a result of this joint effort, different tuned system designs and development strategies have been investigated and developed to control the dynamic behaviour of power ultrasonic piezoelectric systems, as highlighted by Cardoni et al., (2009) and Cardoni et al., (2012). The work presented in this paper deals with: a) the latest advances in the development of airborne ultrasonic technologies for the intensification of lab scale freeze drying processes of interest for the food industry, and b) the first experimental results obtained in a pilot scale installation for the agglomeration of fine aerosol particles produced in severe accidents such as those involving nuclear power plants by high-intensity ultrasonic waves 2. Freeze drying processes assisted by power ultrasound As shown by Gallego-Juárez et al., (1999), Mulet et al., (2003), and García-Pérez et al., (2012), by applying airborne power ultrasound the kinetics of drying processes may be intensified at moderate temperatures in comparison with conventional driers. To overcome the difficulty existing in the direct application of airborne ultrasound in conventional driers, CSIC and the Universitat Politécnica de Valencia (UPV) have designed and developed a new type of ultrasonic drying chamber (UDC). The system consists of three parts: a) a flat platetransducer consisting of a rectangular radiating plate and a piezoelectric vibrator; b) a static structure incorporating reflecting surfaces for distribution of the acoustic field, and c) a drying chamber. Table 1. Electrical response of the flat plate-transducer versus power Power (W)

Phase V-I (º)

Impedance (ɏ)

Current (mA)

Voltage (V)

Frequency (Hz)

10 100 200 300

1 2 4 3

804 650 640 600

110 393 527 692

89 258 366 418

21114 21081 21078 21075

As a result of FEM work performed using Comsol Multiphysics package following a design protocol presented by Riera et al., (2011), the geometry of a rectangular flat-plate transducer structure and positioning of reflectors for acoustic field optimization and drying chamber configuration were defined. The rectangular plate radiator was designed to resonate in a flexural mode with 12 Nodal Lines (NL) (Fig. 1). The radiator material was manufactured using 7075 aluminium alloy and the tuned system operated at a power up to 300 W. The resonance frequency of the

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Enrique Riera et al. / Physics Procedia 87 (2016) 54 – 60

plate-transducer was measured at 21.1 kHz. A high quality factor (Q ~ 17000) was measured with an Impedance Analyzer (HP-4194A). Table 1 shows the main electrical parameters of the plate-transducer at different electrical powers applied to the piezoelectric stack. The diagram of the experimental rig used for vibration testing of the ultrasonic flat plate-transducer is shown in Fig. 2. The transducer response was measured in terms of vibration velocity and current flowing across the piezoelectrics. The experiment was controlled using a Labview application for monitoring operation parameters such as voltage, current, power and system temperature in real time maintaining them within defined limits. The experimental rig consisted of a computer, driving power electronics, the flat plate-transducer, and a measurement and acquisition system. For the characterization of the vibration behaviour of the transducer, a burst excitation was used to investigate the nonlinear dynamic behaviour of the transducer without the influence of thermal effects. A weak softening effect was detected in the ultrasonic system response characterized by less than 4 Hz shifting of the tuned frequency when the maximum voltage was applied (Fig. 3). This means that the transducer had a great stability under high power driving conditions even when a continuous excitation was applied. One of the major problems to be solved was obtaining a directional radiation in air from both faces of the flexurally vibrating radiating plate. To overcome this difficulty it was decided to put in phase the radiation emitted by plate zones vibrating in counterphase. For this purpose, the radiation of each flexing plate sector was separated using parallel walls perpendicular to the radiating surface and located near the nodal lines as described by GallegoJuárez et al., (2015) and shown in Fig. 4. The main characteristics of the directional flat plate transducer with reflectors were measured in an anechoic chamber. Experimental data showed a directivity (-3dB beamwidth) of about 2º in both XZ and YZ planes, together with an electromechanical efficiency of the tuned ultrasonic system in air of about 80 %.

Fig. 1. 3D model by FEM of the flexural mode of the vibrating plate with twelve nodal lines (12NL)

Fig. 2. block diagram of the controller and characterization system used in this work

Fig. 3. vibration velocity frequency response obtained via upward and downward frequency sweeps around the tuned frequency at different applied voltages


Fig. 4. lateral view of the power ultrasonic plate-transducer with reflectors on its front face during mounting process

Fig. 5. geometry model UDC. H is the height

Fig.6. SPL evolution UDC height

In the UDC design the interaction between the flat plate-transducer with the fluid media was taken into account. Hence, a mathematical model was used to allow the analysis of the influence of the main operational system parameters on the sound field distribution, aiming to obtain a high SPL value inside the field for enhanced drying. Fig. 5 shows a geometric model of the ultrasonic drying chamber (UDC), whereas in Fig. 6 an example of the evolution of the SPL versus UDC height is illustrated. Aerosol characterization at the intlet

Aerosol characterization at the outlet

Fig.7. scheme of the full experimental system of PECA-MSAA

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3. Application in environmental pollution: Acoustic agglomeration of fine aerosol particles The main application of the Aerosol Acoustic Agglomeration (AAA) is to eliminate small particles, below 2.5 μm, by their agglomeration and subsequent removal by aerosol deposition, filtering, or retention Mednikov (1965), Shirokova (1973). These small particles represent a hazard for human health and are difficult to trap with conventional methods. This work was aimed at investigating the extendibility of the AAA method previously applied in several environmental applications as illustrated by Gallego-Juárez et al. (1999) to nuclear aerosols produced during severe nuclear accident venting, as described by Allelein et al. (2009), and Shaw and Rajendran (1979). Hence, the expected conditions produced during a hypothetical accident venting in a nuclear reactor were replicated as close as possible following the work by Allelein et al. (2009) wherein the particles of interest had an aerodynamic diameter around 1 μm, as those generated in both Chernobyl and Fukushima accidents, Kauppinen et al. (1986) and Malá et al. (2013). For this reason, an installation schematized in Fig.7 was designed. This installation consisted of two parts, a system for aerosol generation and characterization (PECA), and a Mitigative System Acoustic Agglomerator (MSAA). This latter system consists of a parallepipedic acoustic agglomeration chamber that works in vertical position inside the PECA facility, and a linear array of two power stepped-plate transducers. Both transducers are arranged along the elongated side of the chamber to achieve a high-intensity standing wave field at 21 kHz. MSAA working conditions were optimized to agglomerate tiny particles. For this reason a mean sound pressure level of 155 dB was generated inside the chamber. One of the characterization devices used by the PECA facility is, an Aerodynamic Particle Size (APS) of TSI to measure the outlet aerosol. Again, the disposition of the full facility (MSAA-PECA) can be seen in Fig. 7. The investigated aerosols were produced mixing SiO 2 particles of different sizes or mixing SiO 2 particles of a single size with particles of TiO 2 . Hence, the SiO 2 particles mixed in the aerosol generator to produce a polydisperse aerosol were monodisperse size distributions of around 0.3 μm, 1 μm and 2.5 μm in diameter. The TiO 2 particles had a polydisperse size distribution with diameters between 0.01 μm and 0.05 μm. Table 2 shows the experimental conditions investigated. Table 2. Experimental variables of the test matrix Name Flow Residence time Total Input Mass (s) Concentration 3 (kg/h) (mg/m ) AAA2 AAA6 AAA5 AAA4 AAA1 AAA3 AAA7 AAA8 AAA9 AAA10

100 100 50 12.5 12.5 12.5 12.5 12.5 12.5 12.5

10 10 20 80 80 80 80 80 80 80

25 25 50 200 200 200 200 200 200 200

Mass proportion of SiO ȝP 2

Mass proportion Mass proportion Mass proportion of SiO ȝP of SiO ȝP of TiO 2

2

2

(%)

(%)

(%)

(%)

0 75 75 75 0 50 90 0 50 50

100 25 25 25 100 50 10 75 30 30

0 0 0 0 0 0 0 25 20 0

0 0 0 0 0 0 0 0 0 20

The effect of airborne power ultrasound in the AAA10 test during which the ultrasonic system was switched on and off three times (with an interval of 2 minutes) produced a rise in the number of particles as shown in Fig. 8

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Fig. 8. APS measurements of particle number size distribution of AAA10 test. The white line represents the final time of the test (aerosol generation off, ultrasound off and air flow increased to 200 kg/h).

Fig. 9 shows the changes of the distributions for the AAA4, AAA6, AAA3 and AAA7 tests. In these graphs F1 represents the particle mass size distribution obtained when ultrasound was off and F2 the distribution achieved by ultrasound application. It shows clearly that an increase in flow decreases the agglomeration effect and that higher polydispersion leads to higher sizes. The mass median diameter increases of the distributions measured by the APS go from nearly zero to 37% and are summarized in Table 3. The main effect of ultrasound is to agglomerate the smaller particles with the bigger ones as it is displayed in the changes on the distributions: a lowering of concentration of smaller particles, spreading of the distribution curve and displacement to higher diameters of the bigger particles. a)

b)

Fig. 9. Averaged particle mass distributions at the outlet with and without ultrasound: a) tests AAA4 and AAA6, and b) tests AAA3 and AAA7 Table 3. Mass Median size and GSD changes measured by the APS 0.3 μm particles Name AAA1 AAA2 AAA3 AAA4 AAA5 AAA6 AAA7 AAA8 AAA9 AAA10

¨0HGLDQ %

¨*6'

1.5 0.5 8.4 -4.2 18.6

5.8 -0.3 6.4 -3.0 3.7

4.8 -100.0

2.0 -100.0

1 μm particles ¨ *6' % -12.2 -9.5 -1.8 -0.3 28.0 16.4 34.6 12.7 3.4 4.2 -1.0 0.6 27.9 14.2 -4.2 -4.2 23.3 15.3 37.0 15.3

¨0HGLDQ %

2.5 μm particles ¨0HGLDQ %

¨*6' %

-0.4 4.6

-67.7 -87.3

60


4. Conclusions Two innovative systems for the application of airborne power ultrasound in food drying and environmental processes at lab and pilot plant scales were investigated experimentally. An ultrasonic drying chamber (UDC) was designed and developed to intensify food drying processes. For that reason, a new flat plate-transducer with reflectors was designed, constructed and characterized at high power operational conditions. The ultrasonic assembly operated at a frequency of 21 kHz with a power capacity of 300W showing a very stable response over time. The ultrasonic standing wave field inside the UDC was modelled numerically and SPL up to 172 dB obtained. Another system for acoustic agglomeration of aerosols was designed and tested. A standing wave field at a frequency of 21 kHz with a mean sound pressure level of 155 dB was generated. The treated aerosols consisted of mixtures of monodisperse SiO 2 particles with diameters between 0.3 μm to 2.4 μm and polydisperse TiO 2 particles between 0.01 μm to 0.05 μm. The changes in the particle aerodynamic size distributions indicated an agglomeration effect between the smaller particles with the bigger ones with a median size increase up to 37%. Acknowledgements This work has been partially funded by the Spanish Ministry of Economy and Competitiveness project DPI201237466-C03-01 and the EU-PASSAM project (Grant agreement No. 323217 – Euratom FP7) References Acosta, V., Bon, J., Riera, E., Pinto, A., 2015. Ultrasonic drying processing chamber, Physics Procedia (in press) Allelein, H. J., Auvinen, A., Ball, J., Güntay, S., Herranz, L. E., Hidaka, A., Jones, A. V., Kissane, M., Powers, D., Weber, G., 2009. State-ofThe-Art Report on Nuclear Aerosols, Nuclear Energy Agency Committee on the Safety of Nuclear Installations. NEA/CSNI/R, 5 Cardoni, A., Riera, E., Blanco-Blanco, A., Gallego-Juárez, J.A., Acosta-Aparicio, V.M., 2009. On nonlinear dynamics of plate-transducers. In IEEE International Ultrasonics Symposium, Rome, September. Cardoni, A., Riera, E., Blanco, A., Acosta, V., Gallego-Juárez, J.A., 2012. Modal interactions in ultrasonic plate-transducers for industrial applications. Proc. Inst. Mech. Eng. C J. Mech. Eng. Sci. 226, 2044-2052.Gallego-Juárez, J.A., Rodríguez, G., Gaete, L., 1978. An ultrasonic transducer for high power applications in gases. Ultrasonics 16, 267-271. Gallego-Juárez, J.A., Riera-Franco de Sarabia, E., Rodríguez-Corral, G., Hoffmann, T. L., Gálvez-Moraleda, J. C., Rodríguez-Maroto, J., GómezMoreno, F. J., Bahillo-Ruiz, A., Martín-Espigares, M., Acha, M., 1999. Application of acoustic agglomeration to reduce fine particle emissions from coal combustion plants. Environmental Science & Technology 33 (21), 3843-3849 Gallego-Juárez, J.A., Rodríguez, G., Acosta, V.M., Riera, E., 2010. Power ultrasonic transducers with extensive radiators for industrial processing. Ultrasonics Sonochemistry 17, 953-964. Gallego-Juárez, J.A., Rodríguez, G., Acosta-Aparicio, V.M., Riera, E., Cardoni, A., 2015. Power ultrasonic transducers with vibrating plate radiators (Chapter 7) in “Power Ultrasonics”. In: Gallego-Juárez, J.A., and Graff, K.G., (Ed.). Woodhead Publishing, Cambridge, pp.159. Kauppinen, E.I., Hillamo, R.E., Aaltonen, S.H., and Sinkko, K.S., 1986. Radioactivity Size Distributions of Ambient Aerosols in Helsinki, Finland, during May 1986 after the Chernobyl Accident: Preliminary Report, Environmental Science & Technology. 20 (12), 1257-1259 Mednikov, E.P., 1965. Acoustic coagulation and precipitation of aerosols, Consultants Bureau, New York 0DOi+5XOtN3%HþNRYi90LKDOtN-6OH]iNRYi03DUWLFOHVL]HGLVWULEXWLRQRIUDGLRDFWLYHDHURVROVDIWHUthe Fukushima and the Chernobyl accidents. Journal of Environmental Radioactivity 126, 92-98 Riera, E., García-Pérez, J.V., Cárcel, J., Acosta, V.M., Gallego-Juárez, J.A., 2011. Computational Study of Ultrasound-Assisted Drying of Food Materials (Chapter 13) in “Innovative Food Processing Technologies: Advances in Multiphysics Simulation”. In Knoerzer, K., Juliano, P., Roupas, P., and Versteeg, C., (Ed.). IFT Press and Wiley-Blackwell, Chichester, pp. 265. Riera, E., González-Gómez, I., Rodríguez, G., Gallego-Juárez, J.A., 2015. Ultrasonic agglomeration and preconditioning of aerosol particles for environmental and other applications (Chapter 34) in “Power Ultrasonics”. In: Gallego-Juárez, J.A., and Graff, K.G., (Ed.). Woodhead Publishing, Cambridge, pp.1023. Shaw, D.T., Rajendran, N., 1979. Application of Acoustic Agglomerators for Emergency Use in Liquid-Metal Fast Breeder Reactor Plants. Nuclear Science and Engineering Volume 70, Number 2, 127-134 Shirokova, N.L., 1973. Aerosol coagulation, in Rozenberg L.D., Volume 2, Physical Principles of Ultrasonic Technology, Plenum Press, New York, 475-539